1. Field of the Invention
This invention relates to a force limiting device for a valve actuator rod and, more specifically, such a device which produces a maximum tensile force thereon less than would normally be produced on a piston of a valve actuator associated with the actuator rod when the actuator has located the valve in a backseated position.
2. Description of the Prior Art
There have heretofore been utilized a number of means for closing large valves in the feedwater and steam systems of power plants. It is presently felt that a valve actuator incorporating a hydraulic cylinder and a gas accumulator affords an excellent means for rapid closure of these valves. Hydraulic oil is directed to a piston within the hydraulic cylinder to open the valve in opposition to a precharged source of high pressure gas which acts on the other side of the piston. A charge of high pressure gas is maintained in an accumulator to insure a sufficient quantity is available to act on the piston to rapidly close the valve when the high pressure is relieved.
While this system produces adequate force for opening and closing the valves, it has been found that the dimensions of such actuators and the pressures required are capable of producing a large tensile force on the actuator rod when the valve is in an open, backseated position. Accordingly, there have heretofore been utilized various force limiting devices in the form of couplings between the actuator rod and the valve stem to protect the valve during backseating. While these devices have reliably limited the tensile force, they have required an additional element and thus an added expense to the valve-actuator configuration. There remains a continuing need for a simple, reliable, inexpensive means for utilizing such a valve actuator while insuring the valve will not be damaged during backseating.